In communication systems a modem is used to convert (modulate) digital signals generated by a computer into analog signals suitable for transmission over telephone lines. Another modem, located at the receiving end of the transmission, converts (demodulates) the analog signals back into digital form. Typically, the communication path used by such systems has various limitations, such as bandwidth. As a result, there are upper practical limitations that restrict the quantity of information that can be supported by the communication path over a given period of time. Various modulation schemes have been proposed that effectively increase the information handling capacity of the communication path as measured against other modulation techniques. For example, sixteen-point quadrature amplitude modulation (16-QAM) provides a constellation of modulation values that are distinguished from one another by each having a different combination of phase and amplitude, where each constellation point represents a plurality of information bits.
Due to the changing amplitude from QAM-symbol time to QAM-symbol time, QAM symbols in a QAM communication system require linear amplification to be able to accurately distinguish one QAM symbol at one amplitude level and another QAM symbol at some other amplitude level.
In QAM communications systems, both the average power and the peak to average power ratio (referred to as peak to RMS power ratio) change for different QAM indexes. In particular, the average power and the peak to RMS power ratio is small for low QAM indexes (4, 16-QAM) and higher for large QAM indexes (64, 256-QAM). The difference in average power can cause a deviation of over 1 db in transmitter power, which could lead to exceeding transmission power spectral density levels for a high QAM index (e.g. for 64, 256-QAM) or transmitting too little power for a lower QAM index (e.g. for 4, 16-QAM).
In the prior art, QAM communication systems with high indexes have generally ignored small power level differences due to different average power levels. However, this is unacceptable in QAM systems that must operate in a range from 4-QAM up to 256-QAM and higher. The difference in transmitter power level between 4-QAM and the higher indexes is quite significant and would cause too little power to be transmitted with 4-QAM if left uncompensated thereby unnecessarily reducing transmission reach over an asynchronous digital subscriber line (ADSL), for example.